Particularly in medical research, various imaging methods are used in the analysis of tissue structures and brain signals. Methods used in different applications include magnetic resonance imaging (MRI), simply referred to as magnetic imaging, which is applicable to different parts of the body, and magnetoencephalographic imaging (MEG), which means the measurement and analysis of magnetic fields generated by the electric activity of the brain.
It is typical of the imaging methods that a large set of measurement channels and related measurement sensors are needed therein. The sensors must be very low-noise and situated close to the object to be measured. The typical noise level of a measurement sensor measuring a magnetic field is of the order of a few femtoteslas. It is characteristic of a measurement situation that the flux densities to be measured are very low (for example of the order of 10 . . . 1000 fT), and the external interference fields prevailing in the measurement situation may be quite large in comparison to the flux densities, even of the order of 1 . . . 10 μT. The estimation of the portion of different interference signals in the overall measurement signal and the elimination of the effect of interferences from measurement results is thus extremely essential in multichannel biomagnetic measurement methods.
In the prior art, interferences have been compensated for by using for example a so-called reference sensor assembly at a slight distance from the actual set of measurement sensors. Such reference sensors measure a signal that exhibits only the portion of external interference and practically not at all the useful signal originating from the human body to be measured. Then the interference signal can simply be reduced or projected off from the signal measured by the actual assembly. The problem with the reference sensor method is that the interferences are measured from a site different from the actual biomagnetic signals. If the interference field is not uniform, there is a need to extrapolate the interference field data in such a way as to obtain as good an estimate as possible specifically in the area of the set of measurement sensors.
Another known method is to use a compensating set of coils located in the vicinity of a measurement site to generate a magnetic field compensating for interferences in the area of the set of measurement sensors on the basis of measured interference field information. The problem in this method is to generate the compensating field in such a way that the interference appearing in the area of each measurement sensor can be sensor-specifically canceled even in the case of a non-homogeneous interference field.
A third prior art method is to locate the measurement sensors within a magnetically shielded room (MSR), whereby interferences originating from the environment can be significantly damped. The shielded room typically consists of many superposed metal layers (for example aluminum or suitable metal alloy can be used) constructed in such a way that the interference signals are damped substantially over a large frequency band in such a structure. The shielded room can be fixedly built in a desired site, which is quite an expensive solution, or it may be structurally lighter and built from elements in a desired space and is later moveable to a new site, if necessary. The problem of shielded rooms in minimizing the effects of interferences is that the walls of the shielded room itself may contain magnetic materials and thus they may act as an independent source of magnetic interference signals. A light shielded room is additionally susceptible to vibration, which may further generate an additional magnetic source. Perhaps the most important restriction of shielded rooms, however, is that they do not damp interferences generated within the room. This is a significant problem particularly in clinical MEG studies.
A calculatory method used in the analysis of measured signals is the so-called Signal Space Separation method (SSS method), which is discussed for example in patent publication FI 115324. The SSS method is currently quite amply used in the art. It is a calculatory method for separating multichannel measurement signal information, on the basis of the locations of the sources, into various signal bases, i.e. subspaces that are linearly independent of one another. The SSS is purely based on the geometry of the sensor assembly and natural laws. The calculation according to the principle of the SSS is begun by Maxwell's equations representing the relations of electric and magnetic fields. In the SSS method, it is possible to separate the magnetic fields generated by the useful sources (such as the brain) and the magnetic fields originating from external interference sources. In other words, series developments are calculated in the SSS method using division according to sources located in different sites. It may be referred to as a source modeling method for the multichannel measurement signal in a volume where the magnetic fields to be determined are irrotational and sourceless. The SSS method does not need advance information about the types or locations of the different signal sources but functions correctly in the cases of different types of signal sources, also when examined as a function of time even when the location and/or intensity of the sources changes. In the calculation according to the SSS method, the geometry of the sensor assembly thus plays an important role. Associated to the geometry is also the fact that, in addition to the location, the position of the sensors significantly affects the measured signal because the magnetic field is a direction-dependent quantity. Calibration of the sensors in this context means that the calculation logic has sufficiently accurate information about the locations and positions of the sensors, i.e. the difference between an estimate used in the calculation and the actual real-world situation is determined as accurately as possible. When the calibration information has been determined, it can be observed in the calculation to obtain more accurate information about the useful signals and, furthermore, more accurate analysis results on the basis thereof. As for the SSS method, it can be stated that a calculated subspace formed by the interferences is produced therein on the basis of the measurement results and, on the basis of this information, the desired biomagnetic signal is more accurately accessed.
The shielding factor of a method or device means the extent to which external interferences can be damped by a desired method or device. With default settings of a typical MEG device, a shielding factor of the order of approximately 20 is reached with the SSS method, i.e. using a MEG device that applies the calculation algorithm of the SSS method, the external interferences can be damped to approximately 1/20 compared with what they would be without the SSS method. After so-called fine calibration, the shielding factor of the SSS method typically increases to a value of 100 . . . 150.
Advantages of the SSS method include that it observes all interferences regardless of time and place. Since the calculation is made independently for each sample, it observes the changing situations regardless of whether the interference sources are changing inside or outside the measurement area, i.e. the shielded room. A problem of the SSS method is that it is sensitive to the above-mentioned calibration errors, i.e., for example, a signal deviation measured by one of the sensors may not be due to interference but a small unrecognized deviation in the position of the sensor.
SSP method, in turn, means a so-called projection method (“Signal Space Projection”), based on projecting off the observed interference subspaces, i.e. components, by removing that dimension in question from the measured signal. The principle of the SSP method has been described, for example, in Uusitalo, Ilmoniemi: “Signal-space projection method for separating MEG or EEG into components”, Medical & Biological Engineering & Computing 135-140, 1997. Interference signals can typically be measured in a situation where the object to be measured is excluded from the space to be measured, i.e. by setting the MEG device to measure without a patient inside the device. In this case, magnetic interference fields exclusively originating from the environment can be measured by the sensors of the device at the moment of that particular examination. After this, expressed in a simplified manner, the most essential interference component(s) (conceivable as a vector) is determined from the measured interference signal and the signal measured in a situation including the useful measurement object is projected after this determination in an orthogonal direction relative to said interference. In this case, the most essential forms of interference can be removed from the actually measured overall signal. A weakness of the SSP method is that part of the useful signal under examination is also projected off, unless it is fully orthogonal to the orientation of the main interference. Another weakness of the projection method is that it is not able to observe timedependent changes in an interference field because the interference subspace is only determined according to the situation of a specific period. In this case, interference sources changing in time are not correctly observed when the measurement is later renewed. An advantage of the SSP is that the calibration errors due to the placement and positions of the sensors can be taken into account by this method.
The main problem of the prior art is that magnetic interference fields caused by external interference sources can be damped only approximately to the 100th part. Even when damped, the magnitudes of interference fields are significantly large compared with the typical biomagnetic fields to be measured. Thus, there still exists a need to damp the effect of external interferences in biomagnetic multichannel measurements in one way or another.
As already stated above, the use of a so-called light shielded room around the measurement apparatus also entails problems. In addition to the possibility that the shielded room itself contains magnetic materials, a lightweight shielded room may also tremble, which may constitute a new magnetic interference source.